Sealing cement for spark plug



United States Patent C) SEALING CEMENT FOR SPARK PLUG KarlSchwartzwalder, Holly, and Arthur V. Somers,

Flushing, Mich., assignors to General Motors Corporation, Detroit,Mich., a corporation of Delaware Application October 6, 1955, Serial No.538,884

Claims. (Cl. 106-85) This invention relates to a sealing cement forspark plugs and the like. The cement has particular advantages forsecuring and sealing the metal centerwire elements into the centerboreof a ceramic spark plug insulator. Thus, the invention will be describedand explained with reference to this type structure; however, it is tobe understood that the invention in its broadest scope is not limited tothis particular embodiment.

For use in spark plugs, a cement should be capable of withstandingrelatively high temperatures, up to 600 F. or even higher, since thespark plug is subjected to such heat conditions both during assembly andlater in service. For example, one of the steps in the assembly ofaircraft type spark plugs consists of brazing the shielding barrel tothe shell, thereby subjecting the insulator centerbore elements totemperatures in the neighborhood of about 600 F. Similar hightemperatures are encountered in service.

An object of the present invention is to provide an improved heatresistant cement composition particularly advantageous for use in sparkplugs.

Another object of the invention is an improved, more durable and morereliable spark plug structure.

In the drawing there is shown a representative spark plug structureincluding a metal shell 2 having concentrically enclosed therewithin atubular-shaped ceramic insulator 4. Within the insulator centerbore isthe centerwire assembly 6, including as elements, a center electrodespindle 8, a conductive glass seal 10, metal contact 12 which isimbedded in the glass seal, a cartridge resistor element 14, contactspring 18 and a threaded contact cap 20. The threaded contact cap 20 issealed within the insulator centerbore by means of cement 22 which isthe subject of the present invention. Thus, in the particular spark plugshown, which is of the resistor type and designed for aircraft use, thecement is used to anchor and prevent loosening by vibration of theresistor contact cap and also to prevent extraneous materials fromentering the resistor well which is formed by the center portion of theinsulator centerbore. In other types of spark plugs, the cement may beused to seal and secure other centerwire elements within the centerbore,i. e., the resistors cartridge, other contact elements, electrodespindle or the like.

In accordance with the invention, the spark plug centerwire element issecured and sealed within the insulator centerbore by a cement whichconsists of the reaction product of calcium aluminate with an aqueoussolution of phosphoric acid, all as described with more particulars inthe following detailed description.

As the raw materials for the cement, solid technical grade calciumaluminate, which may contain calcium oxide as an impurity, and reagentgrade phosphoric acid containing about 86% by weight HaPO, may be used.It is preferable to further dilute the acid with water before mixingwith the calcium aluminate. The following method for manufacturing apreferred ce- ICC ment illustrates the proper mixing procedure: 3 partsby volume reagent grade phosphoric acid are diluted with 4 parts byvolume water, thus resulting in a solution of about 48% by weightphosphoric acid. About 395 grams (300 cc.) of this dilute acid are addedwith rapid stirring and with cooling to 600 grams of solid calciumaluminate. An exothermic reaction results. For small batches, cooling byrunning tap water is sufcient to adequately control the reaction;however, in the preparation of larger batches it is preferable to coolby the addition of crushed Dry Ice to the reaction mix. At no timeduring the preparation of the cement should the temperature of the mixbe allowed to go appreciably above 70 F. The point at which theexothermic reaction ceases and at which the artificial cooling can beterminated is indicated by a change in color from gray to marshmallowwhite. After addition of all the acid and preferably a few minutes ofadditional stirring, the mix is ready for immediate use as a cement, orfor storage.

To store the cement, we prefer to place it in lead tubes which may bekept in a refrigerator at about 40 F. until needed. If the storageperiod is to be longer than two or three hours, it is preferable to keepthe tubes of cement in a deep freeze at about 10 F. Cement stored inthis latter manner maybe kept for an indefinite period.

Calculated out, the preferred cement, made as outlined above, containsthe following proportions by weight: 60% calcium aluminate, 19%phosphoric acid and 21% water. Other proportions may, however, be usedsometimes to advantage depending on the precise application to be madeof the cement. For example, cement made by using by weight phosphoricacid in the above procedure is excellent but somewhat more difficult tocontrol during the very exothermic reaction during mixing. Moreconcentrated acid than this renders the reaction too difficult tocontrol to be useful in production operations. A relatively dilute acidcontaining as low as 25% by weight HaPO,l may be added to solid calciumaluminate to produce a cement somewhat lower in strength but having theadvantage of more easily controlled preparation due to the lesseramounts of heat given olf. In practice we have found that all cementsmade by mixing from 2O to 60 parts by weight aqueous phosphoric acidhaving a concentration of vfrom 25 to 75 lW-Weight H3PO4, with from 50to parts by weight calcium aluminate are advantageous. On aningredientfUFingrEdienb-basis, the cement, prior to drying shouldpreferably contain about 50 to 90 parts by weight calcium aluminate,about l0 to 30 parts by weight phosphoric acid and about l0 to 30 partsby weight water. With lesser amounts of water or greater amountsof\acid, the reaction is more difcult to control, while with greateramounts of water and lesser amounts of acid there is a sacrifice ofstrength in the finished cement. The preferred range is from 57 to 62parts by weight' calcium aluminate, from 15 to 28 parts by weight H3PO4and from l5 to 24 parts by weight water. If desired, inert fillermaterials such as brous mullite, fiberglass, asbestos, alumina or thelike may be mixed with the cement to provide additional strength andadjust the consistency to that desired. Thus, the appended claims areintended to comprehend the inclusion in the cement of such inert llers.

To manufacture spark plugs of the type shown in the accompanyingdrawing, the cement is used in the following manner. A small portion ofthe cement is squeezed from the lead storage tube into the threads ofthe contact cap 20, after which the cap is screwed into the insulator.Excess cement is washed away from the top of the contact cap with waterand any excess water may be removed by blow drying. Then the structureis air dried for about two hours and subsequently baked in an oven atabout 300 F. for about two hours to thoroughly dry and set the cement.The most preferable drying and baking temperatures and times will, of

course, depend upon the precise composition of the cement, the aboveprocedure being that found most suitable for the preferred embodiment ofthe cement containing about 60% calcium aluminate, about 19% phos phoricacid and about 21% water before drying.

Tests have shown that the cements herein disclosed are far superior tothe sodium silicate type cements conventionally used in the manufactureof spark plugs and in other devices requiring a ceramic-to-metal seal.The chief advantages are increased heat resistance and increasedstrength. The cement also facilitates processing since, unlike previouscements, it does not tend to boil out during curing and eliminates thenecessity for lengthy drying periods.

While the invention has been described with reference to preferred andparticular embodiments thereof, it is to be understood that variousmodifications may be made, all within the full and intended scope of theclaims. For example, if desired, cement may be made with a high watercontent and subsequently partially dried before use to remove part ofthe water and thereby adjust the viscosity. Alternatively, additionalwater may be added to the cement in order to reduce viscosity.

' We claim:

1. A cement for spark plugs and the like consisting essentially of thereaction product resulting from mixing from 50 to 90 parts by weight inplace thereof aluminate with about 20 to 60 parts by weight of an aque-UNITED STATES PATENT QFFICE CERTIFICATE OF CDRRECTION Patent No.2,829,063

ous solution of phosphoric acid, said solution having a concentration offrom 25% to 75% by weight phosphoric acid.

2. A cement consisting essentially of the reaction product resultingfrom mixing together at a temperature of not over 70 F. from 57 to 62parts by weight calcium aluminate, from l5 to 28 parts by weightphosphoric acid and from 15 to 24 parts by weight water.

3. A cement consisting essentially of the reaction product resultingfrom mixing about to 90 parts by weight calcium aluminate with about 10to 30 parts by weight phosphoric acid and sufficient water to providesaid cement with a thick viscous texture.

4. A cement consisting essentially of the reaction product resultingfrom mixing about parts by weight calcium aluminate with about 39.5parts by weight of an aqueous solution of phosphoric acid, saidphosphoric acid solution having a concentration of from 25% to by weightphosphoric acid.

5. A cement for spark plugs and the like consisting essentially of thereaction product resulting from mixing about 60 parts by weight calciumaluminate with about 39.5 parts by weight of an aqueous solution ofphosphoric acid, said solution having a concentration of about 48% byweight phosphoric acid.

References Cited in the file of this patent UNITED STATES PATENTS1,556,115 Hoskins Oct. 6, 1925 1,908,636 Langenberg May 9, 19332,339,163 Friedlaender et al. Ian. ll, 1944 2,437,209 Rabezzana Mar. 2,l948 2,574,843 Randall et al. Nov. 13, 1951 2,717,438 Schwartzwalder eta1. Sept. 13, 1955 April l, 1958 Karl Schwartzwalder et al.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 2, line '70,1 for "tube into" read tube onto line 35, strike out"in place thereof" column 3, and insert instead calcium Signed andsealed this 27th dasT of May 1958.

(SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Comnssioner of Patents

1. A CEMENT FOR SPARK PLUGS AND THE LIKE CONSISTING ESSETIALLY OF THE REACTION PRODUCT RESULTING FROM MIXING FROM 50 TO 90 PARTS BY WEIGHT IN PLACE THEREOF ALUMINATE WITH ABOUT 20 TO 60 PARTS BY WEIGHT OF AN AQUEOUS SOLUTION OF PHOSPHORIC ACID, SAID SOLUTION HAVING A CONCENTRATION OF FROM 25% TO 75% BY WEIGHT PHOSPHORIC ACID. 